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Boiler fluctuates between 180-215

New here looking for some help, I’ll do my best be to short but thorough in what I’m going through here.

System is a gas forced hot water system, 4 zone Taco setup
30 year old Slant Fin 100 Series w/Honeywell L8124E triple aquastat
1800sq/ft Raised Ranch house
I know it’s old, but I’d like to get through these last two months that we need heat in New England before making a decision on swapping out to a newer, more efficient system.

Out of nowhere, started hearing whooshing in my pipes and crackling fins throughout the house. Heat still works great, but there were no noises prior and I’ve been in the house about a year.

Hi/Lo is set at 180/140 yet it still climbs to about 210-215 when it finally shuts off. Pressure is always constant at 20psi regardless of temperature. Even with the Hi limit set to 160, the boiler is firing up when the temp is at 180. Should I replace the aquastat($250 gamble)? I ordered and have one sitting here but wasn’t sure if that could be it or I just have air trapped. I cracked one of the zone valves that had the river of water sound running through it and got no air spitting out, straight and steady flow of water. The noise in the pipes has lessened over time but still exists, most of the noise is an occasional bang and crackling fins but again, those are new noises.

No recent work has been done, I can’t find any leaks anywhere. I never hear the autofill adding any water to the system indicating that it is running low. I have a drop ceiling basement so any leak would be completely obvious.

All 4 zones work perfectly and get heat when called on, house stays warm but the boiler runs a lot. Gas bill was pretty high which was why I even bothered to check. Plumber friend over the phone suggested I do the expansion tank, vent, autofill and backflow to see if any of those components are creating air in the system. I’m not opposed to doing that, he’s busy and can’t get here and I’m capable of doing the work but I’m just looking for ideas and picking a direction to go.

Basically I’m wondering if I should try the new aquastat or swap out the expansion tank and other items first.

Thanks in advance.


  • STEVEusaPASTEVEusaPA Member Posts: 4,109
    If you confirm the pressure is correct, and staying at 20, I wouldn't do any of those suggested things. None of them are 'adding air' to the system. Sounds like your friend may not know hydronics.
    First I would check/confirm temperature/pressure with a separate gauge.
    You could try closing the valve to the water feeder to see if it's not letting water creep in. But if it were it would have to be exactly matching the leak to see no change in pressure, and no air in the system, so I don't think it's that.
    It's more likely the aquastat not properly sensing the temperature. Now is it from the aquastat or could there be a build up on the well on the boiler side not allowing it to sense the proper temperature.
    Most of the noise you hear is probably from the extremely hot water, and you don't want it going to steam.
    With multiple zones, you boiler probably bounces off the high limit unless all zones are calling.
    In general, when a boiler shuts off, the temperature do creep up.

    If you get a new boiler, follow the standard advice of the professionals-size boiler to heat loss, and get someone who understands hydronics.
    If you're sticking the zoning, should be an easy job. Just don't use that type of air scoop and get something more modern.
    Pump away from the expansion tank.
  • MightBeBrokenMightBeBroken Member Posts: 7
    Awesome thank you. If the temp and pressure is verified, should I go ahead with replacing that aquastat?
  • STEVEusaPASTEVEusaPA Member Posts: 4,109
    I'd go with a modern electronic one. Overall you should be running at lower temperatures. If one of the zones is the indirect, then you don't need a low limit, but the electronic aquastat will give you circulator hold off, and a low water cut-off.
    And it's cheaper than a conventional triple aquastat.
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 6,940

    Just an alternative in case your going to replace the boiler anyhow and don't want to dump money into an expensive aquastat.

    Buy a Honeywell "Strap on " aquastat. It will be a lot cheaper.Add this in as a second high limit. Mount it on the supply pipe as soon as it comes out of the boiler. It straps onto the outside of the pipe. Wire it to "break" the circuit on a rise in temp and wire the other end to only shut down the burner.....not the valves or circulator

    It's possible you have a defective zone valve end switch that is keeping the burner on
  • MightBeBrokenMightBeBroken Member Posts: 7
    edited February 2019
    Thanks for the ideas. I have no problem spending the money on the aquastat(already did but didn’t open) if there’s even a 60% chance it helps, I’d rather keep it the way it was and just get it back to working as it was. I’m going to get a gauge and make sure mine is accurate. If I call for service and aquastat is the first thing someone does it’s going to cost triple that and that money would be best towards the new boiler fund. I really appreciate all of the help and options

    Also don’t want to sound like I’m robbing the trade with a DIY because ultimately hopefully in the Fall I’ll be hiring someone for replacement, just not ready for that currently
  • SuperTechSuperTech Member Posts: 1,303
    I would definitely replace the aquastat. Which one did you order? If you want to eliminate the possibility of a bad end switch on the zone valves just disconnect the thermostat wires to the aquastat and try putting a jumper wire in its place.
  • MightBeBrokenMightBeBroken Member Posts: 7
    edited February 2019
    I got the same Honeywell L8124E pictured

  • STEVEusaPASTEVEusaPA Member Posts: 4,109
    Keep in mind also the difference between Hi and Lo (once again, you won't need a Lo has to be at least 20°
  • MightBeBrokenMightBeBroken Member Posts: 7
    Just following up, I’ve been so busy I just now replaced the aquastat and the boiler turned off right at 180 for the first time. I’ll see how the next couple hours go but I appreciate the help from everyone, hopefully it’s all set. If this is the fix I’m such an idiot for being lazy and not doing this sooner it probably only took 20 minutes total
  • MightBeBrokenMightBeBroken Member Posts: 7
    Seems to be good...crackling fins and loud pipes seem to have calmed down and temp has stayed at or below 180 turning off when it should
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 6,940
    Sounds like you got it. Good job
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